The present invention relates to a capacitive sensor in particular for attachment to a vehicle for sensing the proximity of the vehicle to other objects, for example to assist when maneuvering such a vehicle.
In recent years a number of luxury cars have been fitted with sensors particularly on the rear of the vehicle to warn the driver of objects so that when the vehicle is being reversed, a collision with unseen or obscured objects can be avoided whilst still being able to position the vehicle close to such objects, e.g. walls, bollards etc.
However, such sensors are complicated and expensive and consequently tend not to be commercially viable for lower cost cars.
Therefore, according to the present invention there is provided a proximity sensor for a vehicle comprising: an innermost guard plate; an outermost sensor plate; and calculation means for determining the capacitance between the sensor plate and ground for providing proximity information.
In a preferred embodiment, the plates are mounted on a vehicle bumper. This allows the plates to be mounted in close proximity to provide a strong output signal. In order to avoid being damaged, both plates are preferably mounted on the inside of the bumper whilst being separated by an insulator. The guard plate is used to shield the sensor plate to ensure that it only detects changes in the outwards direction, i.e. away from the vehicle. The plates preferably extend substantially along the entire length of the bumper including any portion passing around the corner of the vehicle. The sensor and guard plates are preferably provided as elongate strips extending along the length of the bumper. Furthermore, the guard plate is preferably wider than the sensing plate, ideally three to four times wider.
Because of the varying shape of the profile of the rear of the vehicle, the guard and/or sensor plate are preferably non-uniform in shape. In particular, the sensor plate preferably increases in width towards the ends. This provides increased and changing sensitivity toward and at the edges of the vehicle. With a uniform strip the sensitivity may be reduced at the edges because the obstruction can only couple capacitively with one side of the sensor, whereas an object directly behind the middle of the vehicle will couple capacitively with both sides of the sensor. Whilst the shape of the sensor plate is ideally tailored to the shape of the vehicle for each application, the sensing plate may have a simpler generic shape to provide improved sensitivity on all vehicles. For example, the sensing plate may comprise a central portion having uniform width with two end portions or stubs of larger width on each end. Alternatively, the sensing plate may have a central portion of uniform width with two end portions having progressively increasing widths.
The sensors of the present invention are primarily intended to be mounted on the rear of a vehicle to assist a driver when reversing. However, the sensors are also suitable for front or even side mounting, e.g. for avoiding collisions with objects at low-level which are obscured from view below the bonnet. When a vehicle is manoeuvring, either forwards or backwards, there is a danger that the side wings may strike an object if the vehicle is turning at the same time. A front or side mounted sensor could be provided to detect such objects near the wings particularly in front of the front wheels.
Different materials have different effects on the output of a capacitive sensor. For example people, concrete, wood, metal, stone and plants provide a strong output from the sensor whereas other objects can give weaker responses, in particular low mass insulating objects such as plastics. This may be problematic in view of the likelihood of encountering plastic objects such as cones when manoeuvring a vehicle. However, whilst objects of different materials provide different outputs, the relationship between the output from the sensor and the distance between the sensor and an object follows a similar formula (the sensor output increases as a power of the distance between the sensor and the object) for any object. By monitoring the speed of the vehicle it is possible to calculate the rate of change of the output from the sensor with distance from that object. Thus it is possible to obtain an indication of the range to the object which has been detected that is independent of the magnitude of the output signal from the sensor and thus use this to provide a more accurate indication of the distance between the object and the sensor irrespective of the material of the object being detected.
Therefore, in accordance with another aspect of the present invention there is provided a proximity sensor including means for determining the speed of the vehicle and wherein the calculation means is adapted to utilize this speed information along with the rate of change of the output to provide more accurate proximity information.
Although the sensor is sensitive to the presence of the surface along which the vehicle is travelling, because this normally has a constant effect, its effect on the determination of the distance to other objects can be cancelled out. However, where the vehicle is travelling over uneven ground then clearly the variation in the height of the sensor above the ground will give the appearance to the sensor that the ground is moving away or towards it. Furthermore, because most vehicles are supported on a xe2x80x98springyxe2x80x99 suspension, as the vehicle accelerates and decelerates, the vehicle tilts forward and backwards causing the front and rear of the vehicle to rise or fall relative to the ground. Again this can have the effect of appearing to the sensor that the ground is moving away from or towards the sensor. This tilting of the vehicle can lead to the sensor providing incorrect or varying information about the distance to an object. A similar effect can occur when the vehicle travels over objects such as metal drains or manhole covers that have good conductance to ground. Whilst these effects can be minimized by reducing the sensitivity of the sensor, clearly this means that the effective range at which an object can be detected is reduced.
Therefore, according to a further aspect of the present invention, the proximity sensor is provided with a second sensor for providing information about the ground clearance. This information can be used to correct the output from the main sensor allowing the sensitivity of the sensor to be maintained.
Whilst the second sensor is preferably a capacitive sensor arranged in a downward facing configuration, this is not essential and any manner of sensor which is capable of providing information about the height of the main sensor above the ground is adequate. Capacitive sensors are particularly preferable for detecting the presence of objects such as manhole covers which are substantially flush with the ground but, like a change in height, are likely to effect the main capacitive sensor.
In addition to the above, where the vehicle is moving towards a slope, the main sensor may be affected by the approaching slope but the ride height sensor, which is not yet over the incline of the slope, has not yet detected the change in height. It is therefore preferable to orient the second sensor such that instead of facing directly downwards, it is arranged to point or detect the height, slightly outwards of the vehicle to provide an indication of change in the slope of the ground over which the vehicle is travelling sooner so as to correspond more closely with the detection range of the main sensor.